As is known in the art, metal parts having opposed substantially planar and parallel surfaces may be welded together by a process described as solid state welding. A method of solid state welding is disclosed by U.S. Pat. No. 6,637,642 (Lingnau).
As an example, a conventional method of solid state welding two tubular workpieces together is illustrated in FIG. 1. (As will be described, the balance of the drawings illustrate the present invention.) Two metal workpieces to be welded together using the solid state welding method of the prior art are identified in FIG. 1 as “W1” and “W2”. Typically, the workpieces “W1” and “W2” include open ends having substantially planar surfaces facing each other. After the ends of the two workpieces are heated to a suitable temperature (i.e., a hot working temperature) by one or more induction coils in a non-oxidizing atmosphere, the induction coil is removed. The ends are pushed together axially, quickly and with some force, and as they engage each other, one or both of the workpieces is rotated about a central axis thereof. This prior art method has been found to be very effective, and has a number of advantages over other known welding techniques.
The induction heating heats a layer of each workpiece, from the open end of the workpiece inwardly, sufficiently to make the material soft enough that it can be plastically deformed when the ends are brought together. The relatively “soft” heated layers at the ends of the workpieces are pushed together axially, and simultaneously, one or both of the workpieces is rotated, subjecting the material in the heated layers to shear stresses. Because they are at the hot working temperature (at a minimum), the contact surfaces of the workpieces tend to adhere to each other when they engage. The shearing action results from the contact surfaces adhering to each other while at least one of the workpieces rotates. It is believed that this shearing action, in a non-oxidizing atmosphere (e.g., a nitrogen atmosphere), tears the microstructure of the metal in the heated layers at the ends of the workpieces “W1”, “W2” apart. Recrystallization of the material in the heated layers at the workpiece ends takes place as the metal cools. It is also believed that the engagement of the heated metal from the workpieces “W1” and “W2” and its recrystallization results in a relatively uniformly fine-grained region that is integrally formed with both of “W1” and “W2”.
This type of bond has a number of advantages over the bonds formed using other welding methods. For instance, the bonds formed using this prior art method have good axial, radial, and circumferential uniformity. The welds provided are substantially free of notches or other irregularities, with smooth profiles on both inner and outer diameters. Post-weld stress relief and post-weld machining are eliminated. Stress risers (e.g., centerline grooves, or toe grooves) are also eliminated. Also, no filler material is needed.
As is known in the art, where the pipe has a larger diameter and/or a thicker wall, multiple segmented induction coils may be used, in order to provide sufficient heat. For the purposes hereof, a reference to “the” induction coil will be understood to refer to one and/or more than one induction coil, depending on whether a multiple segmented induction coil or some other arrangement involving a number of induction coils is used.
As can be seen in FIG. 1, the workpieces “W1”, “W2” may be, for example, at least partially tubular, e.g., metal pipe. The workpieces “W1”, “W2” have respective bodies 10A, 10B with substantially planar end surfaces 12A, 12B at their respective ends 14A, 14B. The workpieces “W1”, “W2” are positioned so that the respective end surfaces 12A, 12B are facing each other. The bodies 10A, 10B are tubular and substantially cylindrical, and respective axes 16A, 16B defined by the bodies 10A, 10B are substantially aligned. The surfaces 12A, 12B and at least parts of the bodies 10A, 10B proximal to the ends 12A, 12B are heated by passing current through the induction coil (not shown in FIG. 1) that is located in a gap 18 between the respective ends 14A, 14B. As is well known in the art, the heating takes place in a non-oxidizing atmosphere. Once the surfaces 12A, 12B have been heated to the hot working temperature of the workpieces “W1”, “W2”, the induction coil is removed from the gap 18, and the workpieces “W1”, “W2” are quickly rammed together, i.e., the workpiece “W1” is moved in the direction indicated by arrow “A”, and/or the workpiece “W2” is moved in the direction indicated by arrow “B” in FIG. 1. When the workpieces are brought together, a selected one of the workpieces is rotated about its axis in the direction indicated by arrow “C” in FIG. 1, to provide a favorable microstructure, as described above. Alternatively, both of the workpieces may be rotated in opposite directions about their axes when the workpieces are brought together, to provide the desired microstructure.
From the foregoing, it can be seen that the conventional solid state welding technique is suitable only where one or both of the workpieces can be rotated. However, in certain circumstances (e.g., where long segments of pipe are to be joined together, for instance, when laying a pipeline), rotation of one or both of the workpieces is not feasible.